Heaters that incorporate high frequency microwave technology are designed to increase the speed and energy efficiency of coating, curing, drying and processing a range of materials.

Although manufacturing as a whole has changed quite a bit over the years with automation, robotics and complex processing recipes, heating -- for the most part -- has lagged behind. The main heat sources today are gas/hot air and infrared. Widely used to process all kinds of materials, these heaters transfer energy by convection (hot air) or radiation (infrared). In both cases, heat transfer is achieved by the interaction between heated air particles (for hot air) or the electromagnetic energy (infrared) and the surface of what's being processed. In the case of hot air or gas heating, only some of the energy heats the surface and much of it bounces off (figure 1A). Penetration to any depth occurs molecule by molecule via thermal conductivity. This is the “brick wall” faced so often when trying to increase cycle time. Greater speed needs higher heat, which can result in overheated surfaces and uneven internal heating.

In the case of infrared heaters, this heat radiation has a very high frequency and usually penetrates only a fraction of a micron (figure 1B). If deep heat penetration is important to the process, you may need to wait while heat is transferred inside by thermal conductivity. To achieve uniformity, heaters may need to be kept at a distance, which can reduce process efficiency and cause run-off heat problems.

Figure 1. Energy is transferred by convection (A), which is the interaction between heated air particles and the surface of what's being processed. Infrared heating (B) has a high frequency and typically penetrates a fraction of a micron. Low frequency microwave (C) can heat material volumetrically and uniformly with minimal temperature difference between the surface and inner layer but with low interaction efficiency, especially when the material's dimensions do not correspond proportionally to the wavelength. High frequency microwave heating (D) uses concentrated high frequency microwave, ranging between 10 and 200 GHz, to provide localized and exclusive heating.

It makes sense that an energy source that can penetrate such as microwave might be a solution. But microwave's success in the home has not been matched in the plant environment. This is because of frequency. Low frequency microwave can heat material volumetrically and uniformly with minimal temperature difference between the surface and inner layer. However, low frequency means low interaction efficiency, especially when the material's dimensions do not correspond proportionally to the wavelength (figure 1C). For example, using 2.45 GHz standard industrial microwave (with a wavelength of more than 5") to heat a glass sheet measuring a fraction of an inch, efficiency of less than 15 percent is all that is attained. Using this kind of microwave on an industrial scale is cost prohibitive and complicated, requiring in many cases microwave generators of more-than-megawatt power. To benefit from the high power density and short heating time offered by microwave, the energy uniformity should be no lower than 3 percent to 4 percent to avoid an additional temperature differential. Achieving this kind of uniformity for conventional industrial microwave is extremely difficult, bordering on impossible.

If the goal is high efficiency, speed and uniform heating, one solution is concentrated high frequency microwave, ranging between 10 and 200 GHz. Here, efficiency can increase to as much as 80 percent and uniformity to 1 percent to 2 percent over large areas (figure 1D). High frequency microwave became available with the development of the gyrotron. Adding directional and focusing capabilities to this device by shaping the energy into a beam created a microwave heater that can be used in an industrial processing environment.

Figure 2. A gyrotron used for high frequency microwave heating generates a microwave beam from a few kilowatts to more than 100 kW and a diameter around 0.4" (10 mm).

High Frequency Microwave Beam

The gyrotron is a 3.3 to 6.6' (1 to 2 m) long, 134 to 214 lb (50 to 80 kg) metal tube (figure 2) that generates a microwave beam from a few kilowatts to more than 100 kW and a diameter around 0.4" (10 mm). Using metal mirrors, the beam can be focused, spread over a surface, directed or even split -- all with heat density where it is focused and no heat where it is not. Customized work chambers offer required shielding and can be configured for large and small products for both conveyor and batch lines. Because the microwave beam can be split, one gyrotron can process products from several sides simultaneously or provide thermal processing for several production lines at once.

Because of the high frequency microwave's way of heating, a range of capabilities are possible in thermal processing. Selective heating of one material or layer while leaving others cool, focusing energy only where needed, and heating one layer or material through an adjoining layer lead to advanced product formulations.

Fast heating is the natural outcome of a beam that generates high heat density and has the ability to penetrate. With high frequency microwave heating, a heat rate of hundreds or thousands of degrees per second can be achieved by a beam controlled at the speed of light. Therefore, curing temperatures of 320 to 392oF (160 to 200oC) can be reached in a fraction of a second, or ceramics can be melted in a just few seconds. Such heating speeds also mean the ability to conduct selective and local heating before thermal conductivity can spread the heat to the adjoining layers (figure 3).

Today's manufacturing processes demand a lot from material capabilities with enhancements such as composite formulations, complex shapes, multilayer processes and ever-increasing production speeds. The gyrotron beam combines the focusing ability of a beam (like lasers or e-beams) with the volumetric heating capabilities of microwave, providing fast, efficient thermal treatment of almost any material. Plastics, ceramics or glass, for example, can be joined without degrading their properties. Or, a coating can be selectively melted or parts can be overheated for bending.

Figure 3. The fast heating speeds of high frequency microwave allow it to selectively heat localized areas or layers before thermal conductivity can spread the heat to the adjoining layers.

Specific Applications

The gyrotron is a heating option where increased productivity and local or exclusive heating is desirable. It is suitable for situations when rapid heating is needed, maintaining material integrity is a must, or where saving energy or environmental conditions are important. Specific applications that might benefit from using the high frequency microwave heating include:

Adhesive Processing. High frequency microwave can be used for exclusive processing of adhesives, curing adhesives between layers of foam or textiles, or joining metals with polymer adhesives.

Joining. High temperature solders and frits can be processed in the same way as adhesives for joining tempered glass, ceramics, metals and dissimilar materials to one another. High quality joints can be achieved without degrading material properties.

Coatings. High frequency microwave can be used to heat a coating on a substrate without significantly heating the substrate. Examples include metallizing plastics and glass, coating films, firing decorative coatings on glass, and curing or melting protective coatings.

Drying. Well-suited for drying porous materials where liquid is trapped inside, high frequency microwave penetrates and interacts only with the liquid, accelerating drying speeds. Solvents also can be safely and rapidly dried in the presence of an active airstream without scrubbers.

Melting. Local melting of glass, ceramics and polymers (bending/forming), sealing electronics and packaging is possible.

Glass Processing. Glass is heated rapidly with high frequency microwave, giving the opportunity for instantaneous sterilization, tube parting without sodium contamination, hard glass/quartz cutting, and glass lamination without autoclaving. Other examples include automotive and window glass processing, bending, and low-E and decorative coating application.

High frequency microwaves such as the gyrotron beam offer a range of capabilities that have not previously been available in thermal processing. Additionally, the technology delivers volumetric uniform heating capability for material thicknesses ranging from dozens of microns to dozens of inches. PH

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